mesh
Table of content
OpenSAND allows the user to choose a mesh architecture, where terminals can communicate with each other without the need of passing through the gateway. The picture below shows an example of such topology.
This type of architecture uses a regenerative satellite that serves both as a relay and switch between the gateway and the terminals: on return channels, messages from the STs are sent to the satellite to be relayed to the proper gateway; on forward channels, messages from the GWs are sent to the satellite to be relayed to the proper terminals. For messages going from one terminal to another, the satellite also performs the necessary switching: the satellite will desencapsulate the arriving packets, check the destination, and redirect the packet to the destination.
This means that a packet going from, for example, ST2 to ST1, will avoid the delay of passing though the gateway, and will be switched at the satellite (step 1 in the picture). This whole exchange amounts to only one RTT, which is around 0.5s in GEO links.
The use of a regenerative satellite also affects the choice of forward and return standards and encapsulation protocols.
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Mesh topology (regenerative satellite) is an advanced mode operation available in OpenSAND. It can be selected from the Configuration tab, when the Advanced mode is active (via Simulation in the menu bar).
When using regenerative mode, DVB-S2 is used on downlinks (from the satellite to terminals/gateways), and DVB-RCS2 in uplinks (terminals to satellite). Several encapsulations protocols are supported, depending on the standard used on each link.
The choice of a mesh topology impacts OpenSAND functioning at many levels, since it is a parameter that affects all three components (STs, GWs and SAT), and their interactions.
One main consequence of the use of this topology (and of a regenerative satellite), is the architecture of the satellite. In regenerative mode, satellites are composed of four of the blocks that form OpenSAND: the Satellite Carrier block, the Physical Layer block, the DVB block, and the Encapsulation block.
Since packets are routed on the satellite, packets must be encapsulated/desencapsulated on-board.
Other features affected by the type of topology include the routing (which is performed on the satellite), the transmission of attenuation measurements, and the type of standard used on forward and return links. The impact (design-wise) of the two different topologies is detailed on each of these features page.
⚠️ Regenerative satellite mode (mesh topology) has not been exhaustively tested with DVB-RCS2, and its correct functioning is not guaranteed.